From what I see, though, one of the big issues is that when you get to be the size of Facebook or Twitter, it HAS to be about making money. Who is going to pay for your servers and who is going to pay your employees who work on the site full-time? Once you hit critical mass, in order not collapse under your own weight, you need to protect your monetary interests and that means closing off access to competing services.
Now, in the past, this wasn't as much of an issue because people actually paid for things and/or the advertisements covered costs. Today, the bottom has fallen out of the advertising market and no one wants to pay for anything anymore. I have friends that think Flickr's $25/year pro account is a rip-off. I think that's a *steal*.
The ecosystem of the web today is full of freeloaders and "entrepreneurs" who are trying to make a quick buck (via VC or getting bought, primarily) rather than trying to build awesome new products that people would actually want to pay for. No one wants to build companies anymore, they just want to build windfalls.